1. Field of the Invention
The present invention relates to a transparent conductive film that is excellent in electrical conductivity and durability and useful for display elements and solar cells. In particular, the invention relates to a conductive film comprising conductive fine particles adsorbed on a support, a conductive film comprising a transparent conductive layer such as an ITO layer provided on a support, and a conductive film comprising a conductive polymer film, instead of the ITO layer, provided on a support.
2. Description of the Related Art
Image displays such as a liquid crystal display (LCD), plasma display (PDP) and electro-luminescence (EL) element have been widely used in many applications such as televisions, computers and various mobile devices that have spread in recent years, and development thereof is making remarkable progress. At the same time, general use of solar cells achieving higher performance is increasingly demanded with the trend to avoid using energy from fossil fuels for the sake of the global environment.
Conductive films are used for such display elements and solar cells. Among conductive films, a transparent conductive film is preferably a conductive film that exhibits high electrical conductivity and high transmittance in a visible light region, specifically transmittance of above 80% in a wavelength region of 380 to 780 nm.
Although metals such as Au, Ag, Cu or Al have been used for producing the conductive film by forming the metals into a foil with a thickness of about 3 to 15 nm, the metal foil has drawbacks including large absorption and insufficient strength. In recent years, as a transparent conductive film, a so-called ITO film having low electrical resistance has been produced by depositing on a glass support indium oxide (In2O3) containing tin as a dopant, and has been widely used as an electrode for display elements such as liquid crystal displays. Display elements used for cellular phones and mobiles are becoming lighter by an order of grams, and supports for the display elements are currently shifting from glass to plastic. By using a plastic support, the weight of the display element has been reduced to one half of the weight of conventional display elements, while strength and impact resistance have simultaneously been remarkably improved. However, there has been a problem with ITO conductive films in that the conductive film is likely to be peeled off from the support since the material for the support has been changed from grass to a plastic film. The problem is caused by the fact that a plastic film has a hydrophobic surface and, hence, a lower adhesiveness with respect to a conductive material, such as a metal-based material, as compared with a glass support having on a surface thereof polar groups, such as hydroxyl groups, and hence a high adhesiveness with respect to a conductive material due to undergoing a reaction with the material.
From the standpoint of heat resistance, although the crystallizing temperature of an ITO film is around 150xc2x0 C., a polycarbonate support, which is a highly transparent resin suitable for the support, cannot be heated above 100xc2x0 C. due to its thermal properties. As a result, the obtained ITO film is an amorphous film exhibiting low mechanical strength and high electrical resistivity as compared with a crystalline ITO film. Thus, the produced ITO film is inconvenient for application where a low electrical resistivity is required. In addition, production of ITO films is very costly since the base material, indium, is a highly expensive and rare metal, thus imposing a limit on the amount by which the production cost of the support can be reduced.
In light of this, use of a transparent conductive film mainly comprising a zinc oxide (ZnO) layer has been gradually spreading due to its low cost and stable distribution. If Al and other impurities are added to a ZnO layer, the layer acquires a low electrical resistance comparable to that of the ITO film. Further, as another alternative to the ITO-based conductive film, use of a conductive polymer is known in the art to produce a conductive film. If a conductive polymer is used, a thin film capable of exhibiting conductivity can be produced by a coating method, thus providing an advantage in that the film can be manufactured at low cost. Also, since an electrode produced from a conductive polymer exhibits far more flexibility than an electrode made from an ITO film does, and since it is low in fragility and unlikely to be broken even when the electrode is used for products that require flexibility, the conductive polymer has an advantage of providing products with a prolonged service life, especially when a product such as a touch screen, which requires a highly flexible electrode, is manufactured. European Patent Application No. 440957 discloses a technique for producing such a conductive film by using polythiophene containing a polyanion. However, these conventionally known conductive films have been found to exhibit relatively low scratch resistance, as compared with the ITO film, and to exhibit an insufficient level of strength for practical use in certain kinds of applications. Further, conventional conductive films do not have sufficient adhesiveness with respect to a support, currently failing in displaying their advantageous characteristics of flexibility.
Conductive films, such as an ITO-based transparent conductive film obtained by using metallic materials, are usually manufactured by forming a metallic material layer on a glass support through a vapor phase method, such as vacuum deposition or sputtering. A ZnO-based transparent conductive film is usually manufactured through methods such as sputtering or CVD. However, the sputtering method has a problem in that manufacturing costs are high because the apparatus used therein is expensive, and this method is not suitable for manufacturing a large area film. In the case of the CVD methods, although the manufacturing costs are low because the apparatus used is inexpensive and continuous production is possible, there is a problem in that a formed film having a smooth surface has enhanced resistivity to thereby produce lowered electrical conductivity.
Regarding of which method is adopted from among vacuum deposition, sputtering or CVD, the produced metallic thin film has drawbacks including insufficient strength and low adhesiveness with respect to the support, as well as low abrasion resistance and low durability. In view of improving film strength, a method has been proposed in which conductive metallic fine particles are fixed on the support using a binder for producing a conductive film. This method, however, may possibly reduce electrical conductivity depending on the conditions for fixing the fine particles, because the binder itself has no electrical conductivity. In view of the foregoing, there has been a demand for a conductive film which fulfils requirements of both sufficient strength and high conductivity (i.e., low resistivity).
Considering the drawbacks of the prior art described above, it is an object of the present invention to provide a transparent conductive film which has excellent conductivity and durability and is suitably used for image display elements and solar cells. Another object of the invention is to provide a conductive film which can be manufactured in low cost, has high strength, good adhesiveness to the support even when adhered to a resin support, and high scratch resistance.
The inventors conducted intensive research, in particular investigating high ion-adsorbing property of a graft polymer which is formed on the surface of a support, and found that the graft polymer has a strong ability to adsorb charged particles and allows arrangement and dense line-up of conductive particles. The inventors also found that a conductive film having excellent conductivity and durability can be produced by forming a hydrophilic surface by making hydrophilic polymer chains present on the surface, and further by properly combining such a hydrophilic surface and a conductive polymer, and finally accomplished the present invention.
According to a first aspect of the present invention, there is provided a conductive film comprising a support having introduced to at least one surface thereof ionic groups to which conductive fine particles, having electrical charges so as to be bindable to the ionic groups, are electrostatically bonded.
In the first aspect of the invention, it is preferable that a transparent support having introduced ionic groups to a surface thereof for adsorbing conductive fine particles comprises a surface graft polymer introduced thereto by graft polymerization.
According to a second aspect of the present invention, there is provided a conductive film comprising a support, which includes hydrophilic graft polymer chains, and a layer of a conductive material provided on the support.
In the second aspect of the invention, it is preferable that the conductive material for forming the conductive material layer is a metal or an oxide semiconductor, and the layer of the conductive material is formed by a vapor phase method.
According to a third aspect of the present invention, there is provided a conductive film comprising a support, which includes hydrophilic polymer chains, and a layer of a conductive polymer provided on the support.
In the third aspect of the invention, it is preferable that the hydrophilic polymer contains hydrophilic graft polymer chains.
Although the functional mechanism of the present invention is not elucidated, the following is conjectured. If ionic groups are introduced into the surface of a support, the support surface produces a layer which is densely and uniformly lined-up with conductive fine particles having a charge opposed to that of the ionic group. That is, a surface layer densely lined-up with conductive fine particles can be formed without using any binder, and the thus formed layer exhibit excellent conductivity even if the layer is thin. Since ionic groups on the surface of the support tightly adsorb the conductive fine particles via an electrostatic attraction force due to mutually opposite charges, it may be conjectured that abrasion resistance of the surface would increase to thus produce high durability. The present invention has an advantage that a transparent conductive film can readily be formed by selecting transparent materials for the support and specifying the diameter of conductive fine particles to be adsorbed.
In the present invention, the support has a hydrophilic surface since hydrophilic graft polymer chains are present on the support surface. And since the hydrophilic graft polymer chains contain polar groups, when a conductive material or a conductive polymer is closely arranged to the surface of a support, the hydrophilic polymer chains function to tightly adsorb the conductive material or the conductive polymer via a polar interaction to thereby form a layer which is densely and uniformly lined-up with conductive particles. As a result, it is conjectured that abrasion resistance and scratch resistance are increased even when the layer is thin, thereby achieving high durability. The conductive film of the present invention provides advantages that since a pure conductive layer can be formed without providing any intermediate layer such as a binder layer, the formed layer exhibits high conductivity and transparency due to thinness.
It is also conjectured that when the hydrophilic graft polymer to be used in the invention is a polycarboxylic acid, i.e., a polymer having carboxylic groups (e.g., polyacrylic acid or polymethacrylic acid) or a polysulfonic acid, i.e., a polymer having sulfonic groups (e.g., polystyrene sulfonic acid or polyvinyl sulfonic acid), the hydrophilic graft polymer acts as a polyanion or a dopant of the conductive polymer and creates a strong ionic interaction with the conductive polymer, thereby providing the film with particularly high strength and high conductivity.
The structure of such a surface phase having fine particles and the structure of such a conductive material layer having high strength can be confirmed by observing the surface by means of a transmission electron microscope or an AFM (atomic force microscope).